1. Field of the Invention
The present invention relates to a laminated retardation optical element for use in a liquid crystal display or the like, especially a laminated retardation optical element having the function of effectively compensating for the viewing angle dependency of the optical properties of a liquid crystal cell (liquid crystal layer), being in the form of a thin film, and capable of effectively preventing lowering of contrast that is caused by interfacial reflection. The present invention also relates to a process of producing the same, and to a liquid crystal display comprising the laminated retardation optical element.
2. Description of Related Art
FIG. 12 is an exploded, diagrammatic perspective view of a conventional, standard liquid crystal display.
As shown in FIG. 12, the conventional liquid crystal display 100 comprises a polarizer 102A on the incident side, a polarizer 102B on the emergent side, and a liquid crystal cell 104.
Of these component parts, the polarizers 102A and 102B are so constructed that they selectively transmit only linearly polarized light having a plane of vibration in a predetermined direction, and are arranged in the cross nicol disposition so that the direction of vibration of linearly polarized light which the polarizer 102A transmits is perpendicular to that of vibration of linearly polarized light which the polarizer 102B transmits. The liquid crystal cell 104 comprises a large number of cells corresponding to pixels, and is placed between the polarizers 102A and 102B.
The case where the liquid crystal cell 104 in the above-described liquid crystal display 100 is of VA (Vertical Alignment) mode, which a nematic liquid crystal having negative dielectric anisotropy is sealed in a liquid crystal cell (in the figure, the directors of liquid crystalline molecules are indicated by dotted lines), is now taken as an example. Linearly polarized light that has passed through the polarizer 102A on the incident side, passes, without undergoing phase shift, through those cells in the liquid crystal cell 104 that are in the non-driven state, and is blocked by the polarizer 102B on the emergent side. On the contrary, the linearly polarized light undergoes phase shift when it passes through those cells in the liquid crystal cell 104 that are in the driven state, and the light in an amount corresponding to the amount of this phase shift passes through and emerges from the polarizer 102B on the emergent side. It is therefore possible to display the desired image on the emergent-side polarizer 102B side by properly controlling the driving voltage that is applied to each cell in the liquid crystal cell 104. The liquid crystal display 100 is not limited to the above embodiment in which light is transmitted and blocked in the above-described manner, and there is also a liquid crystal display so constructed that light emerging from those cells in the liquid crystal cell 104 that are in the non-driven state passes through and emerges from the polarizer 102B on the emergent side, and that light emerging from those cells that are in the driven state is blocked by the polarizer 102B on the emergent side.
Discussion is now made on the case where linearly polarized light passes, through the non-driven-state cells in the above-described liquid crystal cell 104 of VA mode. The liquid crystal cell 104 is birefringent, and its refractive index in the direction of thickness and that in the direction of plane are different from each other. Therefore, of the linearly polarized light that passed through the polarizer 102A on the incident side, the light that has entered the liquid crystal cell 104 along the normal to it passes through the liquid crystal cell 104 without undergoing phase shift, but the light that has slantingly entered the liquid crystal cell 104 from the direction deviating from the normal to it undergoes phase shift while it passes through the liquid crystal cell 104, and becomes elliptically polarized light. The cause of this phenomenon is that those liquid crystalline molecules that are vertically aligned in the liquid crystal cell 104 act as a positive C plate when the cells in the liquid crystal cell 104 of VA mode are in the non-driven state. It is noted that the amount of phase shift which light passing through the liquid crystal cell 104 (transmitted light) undergoes is affected also by the double refractive value of the liquid crystalline molecules sealed in the liquid crystal cell 104, the thickness of the liquid crystal cell 104, the wavelength of the transmitted light, and so on.
Owing to the above-described phenomenon, even when the cells in the liquid crystal cell 104 are in the non-driven state and linearly polarized light is supposed to be transmitted as it is and blocked by the polarizer 102B on the emergent side, part of the light that has emerged slantingly from the liquid crystal cell 104 in the direction deviating from the normal to it is to leak from the polarizer 102B on the emergent side.
For this reason, the above-described conventional liquid crystal display 100 has the problem (viewing angle dependency problem) that the image quality at the time when an image is viewed slantingly from a position not on the normal to the liquid crystal cell 104 is apt to be inferior to that at the time when the image is viewed from the front of the display.
To eliminate the viewing angle dependency problem with the aforementioned conventional liquid crystal display 100, there have been developed a variety of techniques up to now. One of them is a liquid crystal display as described, for example, in Patent Document 1 listed below. This liquid crystal display uses a retardation optical element comprising a retardation layer having a cholesteric structure (a retardation layer having double refractivity), where the retardation optical element is placed between a liquid crystal cell and a polarizer in order to provide optical compensation.
In the retardation optical element having a cholesteric structure, the selective reflection wavelength given by the equation of “λ=nav·p” (p: the helical pitch in the helical structure consisting of liquid crystalline molecules; and nav: the mean refractive index of a plane perpendicular to the helical axis) is controlled to be either shorter or longer than the wavelength of transmitted light, as described, for example, in Patent Document 2 listed below.
Further, as described, for example, in Patent Document 3 listed below, a liquid crystal display using a retardation optical element comprising a retardation layer (a retardation layer having double refractivity) made from a discotic liquid crystal, has also been known as another technique of eliminating the above-described viewing angle dependency problem. In this liquid crystal display, the retardation optical element is placed between a liquid crystal cell and a polarizer in order to provide optical compensation.
In these retardation optical elements, linearly polarized light that has slantingly entered the retardation layer from the direction deviating from the normal to it undergoes phase shift, while passing through this retardation layer, to become elliptically polarized light, as in the case of the above-described liquid crystal cell of VA mode. The cause of this phenomenon is that a cholesteric or discotic liquid crystal acts as a negative C plate. The amount of phase shift which light passing through the retardation layer (transmitted light) undergoes is affected also by the double refractive value of the liquid crystalline molecules in the retardation layer, the thickness of the retardation layer, the wavelength of the transmitted light, and so on.
It is therefore possible to eliminate, to a considerable extent, the viewing angle dependency problem with conventional liquid crystal displays by the use of the above-described retardation optical element, if the retardation layer contained in the retardation optical element is properly designed so that a phase difference brought by a liquid crystal cell of VA mode that acts as a positive C plate and a phase difference brought by the retardation layer acting as a negative C plate, contained in the retardation optical element, are canceled each other.
It is noted that it is possible to eliminate the viewing angle dependency problem with liquid crystal displays to a more considerable extent by the combination use of a retardation layer that acts as a negative C plate (i.e., a retardation layer whose refractive indices Nx and Ny in the direction of plane and Nz in the direction of thickness are in the relationship Nx=Ny>Nz) and a retardation layer that acts as an A plate (i.e., a retardation layer whose refractive indices Nx and Ny in the direction of plane and Nz in the direction of thickness are in the relationship Nx>Ny=Nz), as described in Patent Document 4 as listed below, for example.
In the meantime, the above-described liquid crystal displays comprising liquid crystal cells of VA mode encompass a liquid crystal display comprising a liquid crystal cell of so-called multi-domain VA mode in which liquid crystalline molecules are inclined in two or more different directions when an electric field is applied. In such a liquid crystal display, it has been known that, if light that enters the liquid crystal cell of multi-domain VA mode is linearly polarized one, light transmission is decreased, but that, if light that enters the liquid crystal cell has been converted to circularly polarized light by a λ/4 retardation film, decrease in light transmission can be effectively prevented (the following Patent Document 5 and Non-Patent Document 1).
However, in the liquid crystal display as described in Patent Document 5 or Non-Patent Document 1, although it is possible to prevent decrease in light transmission by converting light that enters the liquid crystal cell of multi-domain VA mode into circularly polarized light by a λ/4 retardation film, the contrast is, we found, drastically lowered when a retardation layer that acts as a negative C plate is placed between the liquid crystal cell of multi-domain VA mode and the λ/4 retardation film in order to eliminate the above-described viewing angle dependency problem.
In connection with the above-described background art, on the other hand, the method in which a λ/2 retardation film and a λ/4 retardation film are bonded to each other at a predetermined angle, as described, for example, in Patent Document 6 listed below, has been known as a method for eliminating wavelength dispersion on a λ/4 retardation film. With respect to patterning of a retardation layer, there has been known such a method that a non-patterned λ/4 retardation layer is laminated to a patterned λ/2 retardation layer in order to obtain three-dimensional images, as described in Patent Document 7 as listed below, for example.
In addition, we already filed a patent application relating to a filter substrate that comprises: a retardation layer (containing a nematic liquid crystal as a main component) having the function of converting linearly polarized incident light into circularly polarized light, composed of a λ/2 retardation layer and a λ/4 retardation layer; and a cholesteric liquid crystalline filter having the function of selectively reflecting the light circularly polarized by the retardation layer (see Patent Document 8 listed below). We also already filed a patent application relating to a retardation laminate that comprises a patterned layer of a liquid crystalline material capable of forming a nematic layer (see Patent Document 9 listed below).